The origin of the infrared activity of rock salt is discussed in terms of the free vibrations of the crystal. Experimental data on the Reststrahlen frequencies and on the frequency displacements of scattered radiation are used to explain the spectroscopic behavior of rock salt in the near infrared.
C. V. Raman, "Optics in India: The Spectroscopic Behavior of Rock Salt," Appl. Opt. 1, 222-225 (1962)